@use 'sass:color';
@use 'sass:math';

///
/// Colour helpers
///

/// Get the luminance of a RGB component
/// Formula taken from https://www.w3.org/WAI/GL/wiki/Relative_luminance
/// @param {number} $component - component value from 0-255
/// @return {number} luminance
@function getComponentLuminance($component) {
    $componentSRGB: math.div($component, 255);

    @if $componentSRGB > 0.03928 {
        @return math.pow(math.div($componentSRGB + 0.055, 1.055), 2.4);
    } @else {
        @return math.div($componentSRGB, 12.92);
    }
}

/// Get the luminance of a colour
/// Formula taken from https://www.w3.org/WAI/GL/wiki/Relative_luminance
/// @param {color} $color - hex code of the colour
/// @return {number} relative luminance
@function getLuminance($color) {
    @return getComponentLuminance(color.channel($color, "red", $space: rgb)) * 0.2126 +
        getComponentLuminance(color.channel($color, "green", $space: rgb)) * 0.7152 +
        getComponentLuminance(color.channel($color, "blue", $space: rgb)) * 0.0722;
}

/// Get the contrast ratio between two colours
/// Formula taken from https://www.w3.org/WAI/GL/wiki/Contrast_ratio
/// @param {color} $color1 - first colour for comparison
/// @param {color} $color2 - second colour for comparison
/// @return {number} contrast ratio, from 0 to 21
@function getContrastRatio($color1, $color2) {
    $color1lum: getLuminance($color1);
    $color2lum: getLuminance($color2);

    // use light colour first
    @if $color1lum > $color2lum {
        @return math.div($color1lum + 0.05, $color2lum + 0.05);
    }  @else {
        @return math.div($color2lum + 0.05, $color1lum + 0.05);
    }
}

/// Adjust the lightness of a colour to meet a given contrast ratio against a comparison colour
/// @param {color} $color - the colour to adjust
/// @param {color} $comparisonColor - the colour the colour needs to meet a contrast ratio against
/// @param {number} $ratio - the contrast ratio to meet, from 0 to 21
/// @param {boolean} $debug - set to true to output a debug message
/// @return {color} the adjusted colour
@function adjustForContrast($color, $comparisonColor, $ratio, $debug: false) {
    $tempColor: $color;

    /// lighten or darken the colour until it passes a contrast ratio check
    @while getContrastRatio($comparisonColor, $tempColor) < $ratio {
        @if (getLuminance($comparisonColor) > getLuminance($tempColor)) {
            $tempColor: color.scale($tempColor, $lightness: -0.5%);
        } @else {
            $tempColor: color.scale($tempColor, $lightness: 0.5%);
        }

        @if ($tempColor == white or $tempColor == black) and getContrastRatio($comparisonColor, $tempColor) < $ratio {
            @warn 'Unable to adjust #{$color} against #{$comparisonColor} to meet contrast ratio of #{$ratio}. Best ratio is #{contrastRatio($comparisonColor, $tempColor)}.';
            @return $tempColor;
        }
    }

    @if $tempColor != $color and $debug {
        @debug 'Adjusting #{$color} to #{$tempColor} to meet contrast ratio of #{$ratio}:1 against #{$comparisonColor}';
    }

    @return $tempColor;
}

/// Adjust the lightness of a colour until it meets a target luminance
/// @param {color} $color - the colour to adjust
/// @param {number%} $targetLuminance - the luminance to lighten the colour to
/// @return {color} the lightened colour
@function adjustLuminance($color, $targetLuminance) {
    $colorLuminance: getLuminance($color);

    @if $colorLuminance < $targetLuminance {
        /// lighten the colour until it beats the target luminance
        @while getLuminance($color) < $targetLuminance {
            $color: color.scale($color, $lightness: 0.1%);
        }
    } @else {
        /// darken the colour until it beats the target luminance
        @while getLuminance($color) > $targetLuminance {
            $color: color.scale($color, $lightness: -0.1%);
        }
    }

    @return $color;
}

/// Adjust a link colour to meet a required contrast ratio against a provided background colour
/// @param {color} $foreground - the foreground colour
/// @param {color} $background - the background colour
/// @param {text} $description - text description of the element, used in the debug message
/// @param {number} $ratio - the contrast ratio to meet, from 0 to 21
/// @output A child selector for links
@mixin checkAndFixDescendantLinkColour($foreground, $background, $description: '', $ratio: 4.5) {
    @if getContrastRatio($foreground, $background) < $ratio {
        $adjustedForeground: adjustForContrast($foreground, $background, $ratio);
        @debug 'Adjusting link colour #{$foreground} to #{$adjustedForeground} to meet contrast ratio of #{$ratio}:1 against #{$description} background colour #{$background}';
        a, .ds_link {
            &:not(:focus):not(:hover):not(.ds_button) {
                color: $adjustedForeground;
            }
        }
    }
}

/// Adjust a secondary button colour to meet a required contrast ratio against a provided background colour
/// @param {color} $foreground - the foreground colour
/// @param {color} $background - the background colour
/// @param {text} $description - text description of the element, used in the debug message
/// @param {number} $ratio - the contrast ratio to meet, from 0 to 21
/// @output A child selector for secondary buttons
@mixin checkAndFixDescendantSecondaryButtonColour($foreground, $background, $description: '', $ratio: 4.5) {
    @if getContrastRatio($foreground, $background) < 4.5 {
        $adjustedForeground: adjustForContrast($foreground, $background, $ratio);
        @debug 'Adjusting secondary button colour #{$foreground} to #{$adjustedForeground} to meet contrast ratio of #{$ratio}:1 against #{$description} background colour #{$background}';
        .ds_button--secondary:not(:focus):not(:hover) {
            color: $adjustedForeground;
        }
    }
}